CHAPTER X 

 PHYLUM ECHINODERMATA 1 



The Echinodermata (Gr. echinos, a sea-hedgehog; derma, 

 skin) are " spiny-skinned " animals that live in the sea. They 

 represent the most complex of all radially symmetrical animals. 

 For a long time they were placed with the Ccelenterata in a 

 group called Radiata, but when their structure and life-history 

 had been thoroughly made out, they were found to have closer 

 affinities with the higher Metazoa. 



Five classes of echinoderms are recognized by most zoologists. 

 Besides these there are several groups of fossil forms. 



Phylum Echinodermata. — Starfishes, Brittle-stars, Sea- 

 urchins, Sea-cucumbers, Sea-lilies. Triploblastic, radially 

 symmetrical animals ; usually five antimeres, ccelom well 

 developed; anus usually present; locomotion in many species 

 accomplished by characteristic organs known as tube-feet ; a 

 spiny skeleton of calcareous plates generally covers the body. 



Class I. Asteroidea (Gr. aster, a star; eidos, resemblance) 

 (Fig. 131). Typically pentamerous; arms usually not sharply 

 marked off from the disc; ambulacral groove present. Ex- 

 amples: Asterias, Astropecten, Culcita. — Starfishes. 



Class II. Ophiuroidea (Gr. ophis, a snake; oura, a tail; 

 eidos, form) (Fig. 138). Typically pentamerous; arms sharply 

 marked off from the disc; no ambulacral groove. Examples: 

 Ophiura, Amphiura, Astrophyton. — Brittle-stars. 



Class III. Echtnoidea (Gr. echinos, hedgehog; eidos, form) 

 (Fig. 141). Pentamerous, without arms or free rays; test of 



1 The echinoderms form a very complex, aberrant ccelomate group, and their 

 study may be deferred until later if desirable. 



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